1. Field of the Invention
The present invention relates to an image information processing apparatus and, more particularly, to an image signal processing apparatus for processing an input image signal obtained by reading an image pattern such as a paper document using a combination of a plurality of image sensors.
2. Description of the Related Art
Recently, a color copying machine or copier is known, wherein a scanning-type proximity image sensing unit for optically scanning a document image is constituted by arranging a plurality of linear image sensors in the main scanning direction. A combination of a plurality of color linear image sensors facilitates the manufacture of a scanning-type linear image sensing unit capable of reading a document image of a larger size. It is difficult for, however, for manufacturers to manufacture a special linear image sensor which has an increased length corresponding to a desired document size, and this results in an increase in cost.
Although the color copying machine with the above arrangement of a plurality of color lineage image sensors has an advantage in the manufacture, uniform brightness and color reproduction characteristics of a color image read signal output from the sensing unit cannot be expected due to a variation in sensitivity among image sensor sub-units, a variation in color characteristics, and the like, even if a document image is scanned using uniform light. In order to match an image read signal from each image sensor sub-unit with those of neighboring image sensor sub-units in view of image characteristics and hence to obtain a high-quality color image free from color ununiformity, a complicated compensating circuit which includes a shading compensator and a color phase compensator is used. Such a color compensating circuit becomes more complex in accordance with an increase in the number of image sensors used, and this renders the entire construction of the copying machine much more complicated. As a result, the manufacturing cost of the copying machine is undesirably increased.
In the color copying machine, along with developments in digital circuit techniques and an increase in capacity and decrease in cost of semiconductor memory devices, a color image is normally processed in a digital manner. The digital image signal is converted to a hard copy by a thermal printer or a laser printer. Since such a printer has a limited density expression capability per pixel (called "tone"); a dither method is used to print a half-tone image such as a picture image. However, in dither processing of an image signal, if an original image includes a screen tone image component, moire noise is inherently generated at that portion, and image quality is considerably degraded. If low-pass processing is performed to eliminate the moire noise, an image resolution density is degraded.